The purpose of this study is to identify changes in resting energy expenditure (REE), food intake, growth velocity, body composition, and pulmonary function over six months in patients prescribed DNAse (Pulmozyme) for treatment of the pulmonary disease of Cystic Fibrosis (CF). Growth failure and malnutrition are common clinical features in CF. Previous studies have implicated various components of energy imbalance as the cause of malnutrition and suggested that improved nutritional status can slow the progression of pulmonary disease and increase life expectancy. We hypothesize that DNAse therapy will be associated with a decrease in the elevated REE seen in children with CF, with improved linear growth and body weight velocity, and with improved nutritional status as indicated by increased total body fat and sucutaneous fat stores after six months of treatment. Since the last progress report, we have been conducting 18 month follow-up examinations of the subjects enrolled in this study in order to characterize the changes in bone density over an 18 month period. We do not expect an effect of DNAse therapy on bone mineralization. However, because chidlren with CF are at increased risk for osteoporosis, we have asked the study participants to return for a final assessment of bone density, growth and puberty status. To date, 17 outpatient visits have been completed.